Not all the trucks have this but of those which do (like the one in the picture) -
Notice that the front wheels are actually single wheels while the back wheels have two wheels on each side of the axle (so 4 on each side, on a double axle truck like in the picture).
The bolts that fasten the wheels to the axle need to be as short as possible so in order for them to go through both of the back axle wheels, the inner wheel is facing the bolts with the curved in side facing inward while the outer wheel have the curved side outside. This way the distance is minimal and you can close the nuts on both wheel together.
Again, not all trucks use this mechanism, some don't even have bolts that go through the wheels but use other means to attached the wheels to the axle.